Sunday, March 06, 2016

Are You Seeing Stars? We Are!

Asterisms recently made the news when a giant star sapphire was discovered in Sri Lanka; the owners claim that it is indeed the largest sapphire displaying an asterism ever found. They intend to sell the "Star of Adam" at auction with the stone currently valued at 100 million dollars. But what is an asterism exactly and how does it occur? Where and how often does it happen?

The ''Star of Adam'', weighing in at 1404.09cts is said to be the world's largest sapphire. Photo courtesy of www.bbc.com

An asterism, as the name suggests, is a reflection effect that is seen in some gemstones that takes the shape of a star. It's the result of light reflecting off of the many intersecting inclusions inside the stone. While most gemstones are regularly sought after for their clarity or lack of inclusions, in asterism-bearing gems, it is these very inclusions which give them value. But not just any inclusions can induce this phenomenon; certain conditions are required to produce this:

Diagram illustrating the parallel and long inclusions crossing in different directions to create the asterism effect. Photo courtesy of www.creslagems.com

1) The inclusions are generally:

- Long

- Thin

- Numerous

- Parallel to one another

These can be made up of anything from tube-like cavities, to rutile crystals (as is the case in sapphire). If this were not enough, there needs to be at least two oriented sets of inclusions that intersect in order to produce the star shape; so it takes quite the included material to achieve this. When light hits these inclusions, the reflection effect will occur at a right angle, creating the star-shape that you see both in the diagram above, and the picture below.

An asterism-bearing corundum, with arrows showing the likely location of the inclusions causing the effect. Photo courtesy of a reader, Marine Explorer.

2) The cut of a stone is also important. Ideally, the stone should be cut into a cabochon, or a polished rounded surface. This is because the effect is best seen rolling off of this rounded surface. Further, the cabochon lends its flat base to better display this effect.

A star-ruby displaying the hexagonal growth structure typically seen in ruby. Photo courtesy of www.gemselect.com

Star-diopside with its typical 4-branched star. Photo courtesy of www.gemselect.com

The rarely seen asterism in moonstone. Photo courtesy of www.onlinejeweller.org

So in what stones do asterisms occur? As mentioned earlier, it's the included materials that will display this effect; meaning that this can be seen in quite a few stones: you'll see this generally in rubies, sapphires, spinel, rose quartz (which can even display diasterism-12 branched star), garnet, diopside and in rare cases, moonstone. Stars can also have varied appearances based on the number of branches they possess; This can allow us to distinguish them. For instance, diopside has a 4-branch star whose branches are generally 90 degrees from one another, whereas garnet will display 4 branches at a slight angle.

While synthetic materials in these gemstones have the ability to manifest this phenomenon as well, thankfully they are fairly easy to detect. Naturally occurring asterisms in gemstones will often have a hazy, almost blurred appearance, which is quite normal for anything produced in Nature. By comparison, the asterisms seen in synthetic materials are often too sharp and distinctively fine. It's very much one of those "too good to be true'' scenarios.

In case you were wondering, not all asterism-bearing materials have million dollar price tags (thank goodness!). In the case of star-sapphire for example, prices average anywhere between 1200-1600$ US per carat, when dealing in the finer quality materials. Let us know if you'd like to have one in your arsenal!

Hello Marine Explorer! Thank you for the comment as well as for letting me know of the rightful owner of this picture. Please know that I will make the necessary correction for copyright and appreciate your work incredibly. Diagrams are often much more illustrative than words and this particular image spoke volumes about how asterisms work. Are you a fellow gemologist, or a mineral enthusiast?

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